In softwood trees, such as pine, wood is composed of long (3 to 5 millimeters) tracheid cells that allow movement of the sap. In contrast, hardwoods, such as oak, have two kinds of wood cells: wood fibers and vessel elements. Wood fiber cells are smaller in length (0.7 to 3 millimeters) and diameter than softwood tracheid cells and do not participate in the living tree's fluid transport. The vessel elements, acting as the conduits for movement of the sap, come in various lengths within a tree and among species. Darker wood, called heartwood, is at the center of the tree where xylem cells eventually enter a state of perpetual dormancy and become dead parts of a living tree system (much akin to human hair and fingernails). Heartwood cells add strength and structure to a tree with their tough, fibrous mass. The lighter wood, sapwood, accounts for the annual rings that appear like concentric bands in the cross-section of tree trunks from temperate climates. Latewood (summerwood), the higher density wood cells that form the growth rings, are prized by some guitar makers as hard, stiff soundboard "reeds." Earlywood (springwood) cells, low density cells, form early in the year.